1. Field of the Invention
The present invention relates to an electrophotographic copying apparatus, particularly to a color electrophotograhic copying apparatus.
2. Description of the Related Art
There is conflicting demand for the surface glossiness between a color image copy and a black and white image copy on a transfer paper. Color copying requires high glossiness for color reproduction while black and white reproduction needs none of such a property which may rather impair legibility and other characteristics reproduced.
It is known that the glossiness of a image copy depends upon a fixing process and is associated with toner melting in particular. In general, a fixing device is so constructed as to soften or melt toner resin by heating or by using a solvent, allowing it to permeate into the tissue of a transfer paper so that the toner is fixed on the paper to give a glossy image copy. Such the fixing device may be provided with a heat roller in order to heat the toner. In this type of color copier, the two- or three-color toners on the transfer paper are melt and mixed through the heat roller.
In color copying, if the toner image is melted insufficiently, it must involve layer interfaces (cavities) which scatter incident light. As a result the image copy have no reproducibility of the original. Similarly, unless the toner transferred onto the transfer paper has a mirror surface, the incident light is reflected by diffusion on the toner surface and hardly admitted to enter the toner layer, making the colors on the transfer paper dark and cloudy.
Therefore toner with a low melting point is used to give glossy and vivid colors. Thereby no interfaces are expected to be formed within the layer of the toner image because of complete toner melting. The toner may be completely melted and becomes less viscous enough to give a hardened flat surface when the transfer paper is sent out of the heat rollers.
However, it is known that there occurs the separation of toner in its layer called an offset phenomenon when the transfer paper is fed out of the heat rollers. The offset phenomenon is also explained below along with the method of solving it, using illustrated figures.
FIG. 1a is a view illustrating how the bonding strength is exerted on a transfer paper 70, a heat roller 71, and toner layer 72. FIG. 1b illustrates a view showing the bonding strengths exerted on the transfer paper 70, the heat roller 71, toner layer 72, and oil layer 73.
As shown in FIG. 1a, the utilization of above-mentioned toner with a low melding point raises a problem of an offset phenomenon occurring because the bonding strength F.sub.T within the toner layer 72 is smaller than the bonding strength F.sub.R exerted between the toner layer 72 and the heat roller 71 as well as the bonding strength F.sub.P between the toner layer 72 and the transfer paper 70 (F.sub.T &lt;F.sub.R, F.sub.p).
To prevent the occurrence of the offset phenomenon, a recent electrophotographic copying apparatus capable of color copying is so constituted that application of oil on the surface of the heat roller 71 in large quantities, as shown in FIG. 1b, established a relationship represented as the following equation (1) among the bonding strengths F.sub.1 within the oil layer 73, F.sub.2 exerted between the oil layer 73 and toner layer 72 F.sub.R' between the oil layer 73 and heat roller 71, F.sub.T within the toner layer 72, and F.sub.P between the toner layer 72 and transfer paper 70: EQU F.sub.1 &lt;&lt;F.sub.2 =F.sub.R' &lt;F.sub.T &lt;F.sub.P ( 1)
In other words, the occurrence of an offset phenomenon is prevented by the presence of an oil layer which makes the bonding strength F.sub.R' exerted between the oil layer 72 and the heat roller 71 smaller than F.sub.R as shown in FIG. 1a.
However there is given such a problem that the difficulty in constantly supplying a uniform layer of oil with low viscosity, namely, small bonding strength F.sub.1 within its layer 73. The reasons are why that oil flows; surface energy of the heat roller must be raised to constantly supply a uniform layer of oil; supply of oil necessitates a tank for storage and a roller and blade for application of oil; and oil tends to vaporize when heated, giving adverse effects to other processes.
As a method of solving such a problem, U.S. Pat. No. 3,578,797 is disclosed. According to the patent, a toner image is fixed by allowing it to contact a heating web to be heated and melted, cooling the melted toner to make it relatively highly viscous, and removing the transfer paper from the heating web when the toner thus no longer adheres to the web strongly. It is, however, known that it takes the toner a long time to be heated and melted if not allowed to contact the heating body under pressure because the heat transfers very slowly.
In the Japanese Patent Publication No. 72376 (1990) and No. 162383 (1990), a method of pressurized toner heating is disclosed. The transfer paper is heated and pressurized between a heating and a pressure roller, then conveyed while being allowed to tightly contact a fixing film, and removed from the film after cooled below the melting point of the toner. The fixing film is supported by the heating or pressure roller and other rollers and controlled so as to move at an equal speed. This permits the surface glossiness of a toner image to be established as desired in accordance with the surface coarseness of the fixing film as the sufficiently heated toner image surface is kept contacting the fixing film until it is cooled to solidify.
Notwithstanding, a problem common to such the mechanism to improve glossiness in color copying is that improved glossiness rather cause a disadvantage to black and white reproduction as mentioned above.
In other words, on one hand utilization of toner with a low melting point gives a highly glossy and vivid reproduction if an offset phenomenon is prevented, on the other hand a high glossiness causes the image copy including letters and figures indistinct due to the reflected light in black and white copying if the copier operates under the same conditions.